This invention relates to an apparatus for detecting quickly and accurately a fluid leakage occurring at a position between the upstream and downstream points of a relatively long fluid passage and the position of the fluid leakage as well.
Sometimes a fluid passage such as an oil pipe line may encounter a fluid leakage due to an accidental formation of a hole or a crack in the pipes. Such a fluid leakage possibly grows into a great disaster if the fluid has a low flash point as gasoline or is a very poisonous chemical substance. It is therefore necessary to maintain the pipe line in normal condition to guarantee a continuous transportation of the fluid, to detect a leakage as soon as possible if it should happen, and to locate accurately the position of leakage.
Hitherto known is such a leakage detection apparatus as illustrated in FIG. 1. Such a conventional apparatus is installed in an oil transportation system wherein between a tank 1 at the oil supply station and a tank 2 at the oil receipt station a pipe line 3 is connected so as to supply the oil from the tank 1 to the tank 2 with assistance of a pump 4 at the upstream. Near the pump 4, for example, a turbine flow meter 5 is attached to the pipe line 3 and works as primary flow meter. Similarly near the tank 2 another turbine flow meter 6 is attached to the pipe line 3 and functions as secondary flow meter. Each turbine flow meter comprises, as well known, a magnetic rotary blade to be rotated by the flowing fluid and an electromagnetic coil surrounding the rotary blade. These flow meters 5 and 6 generate pulse signals at pulse rate proportionate to the flow rate of the fluid passing through the pipe line 3.
Unless a fluid leakage takes place, the two turbines 5 and 6 generate pulse signals at the same pulse rate. These pulse signals are passed through gate circuits 8 and 9, the gates of which are simultaneously opened by the output of a gate signal generator 7. Then they are counted by a downcounter 10, and the difference in number between the pulse signals from the gate circuit 8 and those from the gate circuit 9 is detected to be zero in this case. Should the fluid leak from the pipe line 3, for instance, at a position nearer to the flow meter 5 than to the flow meter 6, the turbine flow meter 6 at the downstream generates fewer pulse signals for a given time. As a result, the difference in number of the pulse signals is sensed by the down-counter 10, the count value of which is then supplied to a comparator 11.
Even if the fluid does not leak at all, the pulse rate of each turbine flow meter varies temporarily due to, for example, changes in fluid pressure built up by the pump 4 or in accordance with the kind of the fluid. Due to such variation of pulse rate an erroneous detection of fluid leakage could be made. To avoid such an erroneous leakage detection a warning value is predetermined and compared with the count value of the down-counter 10. Namely, if the fluid leaks from the pipe line 3, the count value of the down-counter 10 surpasses the predetermined warning value, and the comparator 11 gives of a warning signal telling the occurrence of a fluid leakage.
Generally, if a counter counts pulses supplied through a gate circuit, its count value at the end of the predetermined opening period of the gate circuit turns out to be erroneous by .+-.one pulse inevitably because of quantization error. As a result, the erroneous count value of the counter reduces the resolution of the count value with respect to the actual flow rate of the fluid. To elevate the resolution of the count value it may be proposed that the count value of the counter be made larger so as to make the .+-.one-pulse counting error small in proportion to the count value. Further, the resolution of the count value is limited by the pulse rate of the flow meters. The number of pulse signals from the flow meters should therefore be increased so that more pulses might be counted by the counter in a given time, thus diminishing the proportion of the .+-.one-pulse counting error to the whole count value of the counter. However, the pulse rate of a commonly used flow meter as a turbine flow meter can hardly be enhanced, restricted by the mechanical factors of the meter. For example, the pulse signals generated by the conventional flow meter are directly counted to learn the flow rate. For this reason the conventional fluid leakage detection apparatus as shown in FIG. 1 detects a fluid leakage but with a low precision. In order to elevate the detection precision, the gates of the gate circuits 8 and 9 may be opened for relatively long time so that the counter 10 may count more output pulses from these gate circuits 8 and 9. If the counting time for the counter 10 is made longer, however, the fluid leakage cannot be detected quickly, letting the fluid leak from the pipe line 3 in a greater amount than otherwise. Morover, a slow leakage detection may turn out to be an incorrect detection because its precision is affected more acutely than a quick detection by external factors such as the temperature, pressure and density of the fluid. What is worse, a slow leakage detection cannot locate the position of leakage accurately.